1. Field of the Invention
The present invention relates to a frequency synthesizer feasible for a transmitter/receiver included in an automobile telephone system or similar mobile communications system and, more particularly, to a frequency synthesizer capable of reducing spurious components away from the center frequency of an output signal by integral multiples of a reference signal.
2. Description of the Related Art
A frequency synthesizer using a PLL (Phase Locked Loop) has customarily been used as, for example, a local oscillator included in the transmitter/receiver of a mobile communications system, e.g., automobile telephone system. This type of frequency synthesizer has a voltage controlled oscillator (VCO), variable frequency divider, phase comparator, charge pump, and loop filter. The VCO produces the output signal of the synthesizer. The variable frequency divider divides the frequency of the VCO output and feeds the resulting signal to the phase comparator. Comparing the output of the variable frequency divider with a reference signal having a reference frequency, the phase comparator produces comparison outputs and delivers them to the charge pump. In response, the charge pump applies a charge/discharge control signal to the loop filter having a capacitor therein. The charge/discharge control signal controls the charging and discharging of the capacitor. The resulting control voltage signal from the loop filter is fed to the VCO.
The phase comparator operates at a timing reciprocal to the reference frequency fr and, therefore, causes the charge pump to operate every 1/fr period. Although most of the components having the reference frequency fr are filtered out by the loop filter, the components passed through the filter are superimposed on the control voltage signal and modulates the phase of the VCO output. Consequently, spurious signals which are apart from the reference frequency fr by integral multiples appear in the VCO output. This is particularly conspicuous when the frequency of the synthesizer output is switched.
Moreover, even when the output signals of the phase comparator are absent, the control voltage signal falls since the capacitor of the loop filter is discharged by a leak current. The charge pump, therefore, operates every fr period even after the PLL of the frequency synthesizer has converged, resulting in the spurious signals which are remote from the reference frequency fr by integral multiples. Hence, in the transmitter/receiver of a mobile communications system using such a synthesizer, the apparent C/N ratio of nearby channels is degraded, degrading the selectability of nearby channels.